SURFACE RESISTIVITY AND TRIBOELECTRICICATION



SURFACE RESISTIVITY AND TRIBOELECTRICICATION

Triboelectric charge generation by plastic packaging materials is widely believed to be dependent on the surface resistivity of the materials in question. If a material has a low resistivity it is sometimes regarded as having a low propensity for charge generation. This section presents data that contradicts this belief. Surface resistivity and charge generation can not be correlated. However, the belief of a relation of these two parameters persists.

For a material to be "antistatic" it must have a low propensity to generate triboelectric charges. As the following charts show, earlier surface resistivity scales listed an antistatic category. Presently the EIA, ESD Association and Military specifications have dropped any reference to such a relationship. Current standards recognize only three basic resistivities for nonshielding materials:

CONDUCTIVE

DISSIPATIVE

INSULATIVE

According to Webster's Third New Inter- national Dictionary, Triboelectricity is "a positive or negative charge which is generated by friction." Triboelectricity is from the Greek, Tribein, which means: "to rub." On the other hand, "contact charge" is the positive or negative charge generated by first the contact and then separation of two materials. Typically, in ESD work, these two mechanisms are lumped together in the term triboelectrification or just tribo.

Early electrostatic work placed a great deal of emphasis on the relative position of materials in a tribo series. The relative polarity of charge acquired on contact between any material in the series with another was predicted by its location. There is little correlation between the series developed by different researchers due to the very complex nature of the triboelectrification process. One such series could be described as below:

Material Polarity

Quartz positive

Silicone elastomer

Glass

polyformaldehyde

polymethyl methacrylate

Human hair

Ethyl cellulose

Polyamide

Salt, NaCl

Melamine

Wool

Fur

Silk

Aluminum

Cellulose acetate

Cotton

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Steel

Wood

Amber

Copper

Zinc

Gold

Polyester

Polyurethane elastomer

Polystyrene

Natural Rubber

Polyethylene

Polypropylene

Polyvinyl Chloride

Silicon

Polytetrafloroethylene negative

The question of whether or not materials at the positive end will always charge positive when rubbed with or contacted by materials lower in the series is not clear. If electron transfer was the only mechanism for charging, at least for certain material combinations, then such a series would certainly exist. However, instead of a uniform series of materials, some "rings" have been shown to exist. The following tribo ring of silk, glass and zinc is but one example of the inconsistencies in tribo series.

[pic]

Silk charges glass negatively and glass charges zinc negatively, but zinc charges silk negatively. This is the case even though glass is higher than silk and silk is higher than zinc in most tribo series. One may not rely totally on a tribo series to determine the polarity of the charge for the contacting or rubbing together of two materials.

No tribo series may be used to determine the actual quantity of charge resulting from the contacting or rubbing together of two materials. The mechanisms for determining the quantity of charge transfer are extremely complex.

Some of the contributors to the ability or inability of two materials to charge each other are illustrated below. The relative magnitude of the contributions of each is subjective and is not reflected in any academic work. Whether the charging is between two polymers, a metal and a polymer, or other materials, they play vital roles in determining the polarity and quantity of charge.

Surface Physicals

Tacticity (coefficient of friction)

Smoothness

Topology

Viscoelasticity (conformability)

Material Physicals and Chemicals

Morphology (amorphous, crystalline)

Work Function

Energy Level

Fermi Level

Electronegativity (metals)

Purity

Polymer Backbone

Polymer sidegroups

Physical State (gas, liquid, solid)

Molecular Mobility

Temperature

Tribo Series Position

Contact

Time of Contact

Area of Contact

Number of Contacts (repeated contacts)

Type of Contact

rubbing

rolling

point

directional (reversal)

Contamination (surface)

Humidity/water

Material transfer

Surface Reactions

oxidation

reduction

sulfonation

flouridation

Particulate

Greases/oils etc.

While all of the parameters stated above play roles in the triboelectrification process, no one parameter or variation of that parameter dominates the total process. For example, PTFE TEFLON sheet has a very low coefficient of friction but is one of the most aggressive tribochargers. The reasons for this are not well understood. A major factor in TEFLON's charge propensity may be related to its polymer composition.

It is known that solidified pure rare gases ("ideal" insulators) do not contact charge unless they are doped with electronegative molecules.

Surface resistivity does not play a role in the tribelectrification process. It does however, contribute to the material's ability to bleed off any charge which has been transferred. Materials with surface resistivities in the static dissipative range will not retain static charges accumulated by tribocharging if those materials are grounded.

TEST METHOD

The test equipment set-up used to collect the data presented in this section consisted of an electrostatic voltmeter described by Baumgartner in two of his papers before EOS/ESD Symposiums. It is essentially a charge-plated monitor. The fixture utilizes an insulated aluminum plate viewed by a noncontacting electrostatic voltmeter. The output of the electrostatic voltmeter was connected to a storage oscilloscope. The voltages being measured on the aluminum plate were displayed on the oscilloscope for easy reading. With this test set-up, any ESD material can be evaluated for their tribocharging propensity against many materials or surfaces of interest.

The surfaces against which the materials were tested were attached to the aluminum plate of the electrostatic voltmeter assembly. The charge accumulated on the test surface develops a voltage, which can be effectively viewed by the noncontacting voltmeter with little loading. This voltage is either capacitively coupled to the insulated aluminum plate (in the case of insulative or dissipative materials) or directly coupled in the case of metal surfaces.

The materials being tested were stroked vigorously by hand against the test surfaces for 5 seconds. The materials were then abruptly removed from the test surface at the end of a stroke. The peak voltage was recorded. Four stroke and separate sequences were performed and recorded for each test material and test surface. The results were averaged and reported. The technician performing the tests wore wrist straps, an antistatic lab jacket, and antistatic gloves. At no time during the tests were the gloves allowed to touch the aluminum plate. The test surfaces and materials were neutralized prior to each test to remove any precharges. The test surfaces were cleaned frequently with methyl alcohol (except for the textile surfaces).

The test surfaces used for the data in this section were:

Quartz

Glass

Wool

Silk

Aluminum

Steel

Copper

Ceramic Integrated Circuits

Solder Masked Circuit Board

Polyester

Silicon Wafer (polished)

Natural Rubber

PTFE TEFLON

FPE TEFLON

These surfaces were chosen to represent a wide range of materials, which might give an approximate tribo characterization to the ESD materials under test. Even though these represent the full range of most tribo series, they fall short of providing a true estimate of how a packaging material might react to any other material encountered in electronic manufacturing. These are only benchmarks. To obtain an estimate of the tribo charge-generating propensity for any given ESD packaging material, one must test it against those materials it will encounter in the particular application.

TRIBO CHARACTERIZATIONS

Many tests were run on most presently available ESD packaging materials as well as other materials of interest. The following series of characterizations illustrate that surface resistivity and triboelectrification do not correlate.

For these illustrations the following materials were characterized. They are listed in order of surface resistivities.

|MATERIAL |RESISTIVITY |

| |Ohms/Square |

| | |

|Copper Mesh | ................
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